Organosiloxane compositions which cure to elastomeric solids are well known. Typically such compositions are obtained by mixing a polydiorganosiloxane having reactive terminal groups, generally silanol groups, with a silane cross-linking agent reactive with the polydiorganosiloxane, for example an acetoxysilane, an oximesilane, an aminosilane or an alkoxysilane. These materials are frequently curable upon exposure to atmospheric moisture at room temperature.
One important application of the above-described curable compositions is their use as sealants. In use as sealants, it is important that the composition is capable of curing in comparatively thick layers to provide an elastomeric body having a thickness greater than about 2 mm. It is frequently desirable that the composition cures quickly enough to provide a sound seal within several hours but not so quickly that the surface cannot be tooled to desired configuration shortly after application. Among specifically desirable attributes for such compositions are fast surface cure rate, good elasticity of the skin formed and lack of surface tackiness after curing for about 24 hours. Also, it has long been desired to have a clear, translucent or “water white” product which retains its translucency and lack of colour during use.
In order to achieve the desired speed of cure of alkoxysilane cured silicone compositions it has become a practice to employ certain organic titanium compounds as catalysts for the condensation reaction. Some of these titanium compounds are apt to react with methoxysilane to form a white precipitate and this gives a discolouration and restricts the ability of the composition to cure. The titanium compounds most generally preferred for this purpose are those derived from primary or secondary alcohols, for example, isopropyl alcohol and n-butyl alcohol. However, the titanium compounds employed in practice are often inadequate to promote a sufficiently rapid and/or deep cure and it is a practice to employ a chelating agent for example an acetyl acetonate as an accelerator and stabilizer for the titanium compound, which accelerators generally cause a yellow discoloration. Examples of such accelerators are described in the following prior art.
U.S. Pat. No. 3,334,067 describes a method of preparing a one package room temperature silicone rubber by mixing an hydroxylated siloxane polymer with a silane cross-linking agent and a beta-dicarbonyl titanium compound such as bis(acetylacetonyl)diisopropyl titanate. EP0164470 describes an organopolysiloxane fluid containing at least two alkoxysilyl organic radicals and titanium, zirconium, hafnium metal or vanadium oxide esters soluble in the liquid. EP0361803 describes a method for the in situ formation of titanium chelate catalysts in which a silicone sealant is prepared by adding an alkoxy or hydroxy endblocked polydiorganosiloxane, an alkoxy functional cross-linker and a titanate catalyst. In this case the titanate catalyst is a tetraorgano titanate such as tetra isopropoxy titanate and tetra butoxy titanate to which ethylacetoacetate is added. U.S. Pat. No. 4,438,039 describes a titanium chelate catalyst which does not form nodules upon storage and comprises the reaction product of a two stage reaction system. The first reaction was between Ti[OR]4 and the following two ketones
where R22 is said to be halogen substituted or unsubstituted monovalent hydrocarbon radicals, R23 is hydrogen, hydrocarbyl, halohydrocarbyl or acyl R24 is a radical having not  more than eight carbon atoms consisting of hydrocarbyl, halohydrocarbyl, cyanoalkyl or amino groups and R25 is a hydrocarbon radical, amino, ether or a polyether. No examples of halogen substituted hydrocarbon radicals are provided. The resulting products are then reacted with a diol HO—R26—OH where R26 is an alkylene group having 2 to 20 carbon atoms to form the final catalyst. The examples and claims teach that R23 is Hydrogen, R22 and R24 are methyl groups and R25 is an ethyl group.
In EP0802222 there is described a method of improving the adhesion of a room temperature vulcanisable silicone composition comprising a polydiorganosiloxane having at least two alkoxy end groups, a cross linker, filler and a titanium catalyst, for example tetraalkoxy titanates such as tetraethoxy titanate and tetra isopropoxy titanate or chelated titanium compounds such as bis-acetylacetonyldiisopropoxy titanate, with 0.25 to 3 moles of monoketoester per mole of each titanium atom in the catalyst.
EP0747443, teaches the use of a catalyst according to the general formula M[OR]x[OR′]y where M represents a metal having a valency of 4 selected from Group IVB of the Periodic Table, x has a value from 0 to 0.60, y has a value from 3.40 to 4.0 and (x+y)=4, and R′ represents a monovalent, tertiary or branched secondary aliphatic hydrocarbon group and R represents a monovalent linear aliphatic hydrocarbon group having 1 to 6 carbon atoms. The catalyst was preferably tetra (tertiary butoxy)titanate or tetra (tertiary amyloxy) titanate. Such a catalyst used for the preparation of silicone sealants was said to cause fast curing compositions, which do not yellow, without the need for a chelating agent.
The introduction of EP0747443, filed in June 1996, teaches that whilst an acetylacetonate may be mixed and/or reacted with the titanium compound to provide a complex as described in the prior art above, use of these resulting complexes as catalysts for curing silicone products inevitably results in a cured silicone product which has a yellow tint or discoloration.
The technology discussed above is particularly relevant to the preparation of room temperature cure sealants. When compared to well known room temperature vulcanisation oxime cure sealants, typical alkoxysilane cured sealant still have two significant problems, (1) a slower surface cure rate, and (2) a feeling of a residual surface tackiness after the period of time the surface is measured as “tack-free” using the standard Tack-Free-Time (TFT) test method (CTM 095A). Furthermore, possibly as a consequence of the overall mechanism of cure of the system (i.e. diffusion of moisture from the ambient atmosphere into the deep section of a sealant bead), the adhesion to some important substrates like floated glass panes does not build-up perfectly all along a sealant-substrate bond line. This phenomenon may suggest that the adhesive strength of the sealant to the glass is not strong enough for the glazing application envisaged. In the case of a glass or mill finished aluminium substrate, it has been found that with alkoxysilane cured sealants, after a period of 7 days curing the sealant is adhered to the substrate around the edges of a bead of sealant but no significant adhesion can be detected at the interface between the substrate and the centre of the bead. This phenomenon, which is a significant problem with respect to adhesion of, in particular, the above two substrates is generally termed as channel adhesion.
End users are known to consider that a sealant with a residual surface tackiness has several drawbacks, for example, it may be thought that the product will not cure completely and thereby, may lead to the, typically incorrect, belief that such a problem also causes poor bulk properties. Tacky surfaces are more prone to dirt pick up in a dusty environment such as in a factory.
Furthermore, whilst a solution to the problem of yellowing has been sought for many years and has been solved partially by EP0747443, it is still not known how to achieve the characteristics of desired speed of cure and adhesion build-up, particularly on glass and metals together with non yellowing with the titanium based catalyst systems.
The present invention seeks to provide an improved sealant based on alkoxy-functional curative/Group IV B metal ester catalyst by providing both an improved surface cure rate performance and the early adhesive bonding performance of the sealant on glass and typical metals like aluminium, copper and brass as used in the construction industry. It has also been found that contrary to the teaching of EP0747443 the addition of certain chelating agents to catalysts such as those described in EP0747443, results in alkoxysilane-cure silicone compositions which are curable at room temperature in the presence of atmospheric moisture at a desired rate and to a desired thickness and provide a translucent, “water white” cured products without the aesthetically unpleasant yellowing discoloration issue.